DE-OS 37 34 421 discloses a cover for a compensation reservoir, the cover being composed of several parts. The screw thread in this cover is so configured that the ambient air can propagate through the windings of the cover's thread into the external inner space of the cover bottom. To prevent brake fluid flowing within the reservoir from being conveyed through the air duct described and into the environment due to the movement of the vehicle, the edge of the connecting socket is hermetically sealed by means of a seal connected to the cover. It is advisable that the air duct gets into connection to the inner space of the reservoir roughly at the level of the axial center axle of the reservoir because the level differences of the moving brake fluid are most insignificant at this point. To this end, the mentioned publication arranges for an air duct that reaches from the radial external inner space of the cover until the center axle of the cover. The air duct can be covered by the inside of the seal or by a special separate cover plate.